Baseflow Nitrate Concentrations Research Articles

Modeling stream baseflow nitrate concentration in an agricultural watershed using neural network and bootstrap method

The evaluation of water resource vulnerability to nonpoint source pollution in the presence of uncertainty remains a crucial concern. To enhance the accuracy of the assessment and pinpoint the key factors that affect watershed water quality, the integration of an artificial neural network (ANN) into the evaluation process is imperative. The research involved the collection of streams baseflow samples from thirty-eight sites between 1994 and 1999 in the Tomorrow-Waupaca River Watershed in central Wisconsin, USA, with a focus on examining the relationship between nitrate concentrations and a range of environmental and land use variables extracted from the watershed GIS database. This study utilized ANN methodology, combined with a bootstrap technique that employed a random resampling approach from a single input dataset, to simulate monthly stream baseflow nitrate concentrations. The effectiveness and predictive ability of the ANN model were assessed by comparing it to conventional multivariate regression methods. Through the use of ANN, more precise outcomes can be achieved while taking into account the uncertainty associated with the analysis. The findings demonstrated that the ANN outperformed both the multivariate linear regression and nonlinear quadratic response surface models in explaining the variance of stream nitrate and in external prediction consistency. This study also highlighted several key variables, such as the areal percentage of agricultural land and grassland, stream order, and the average slope of the groundwater flow path, that significantly impacted the stream baseflow nitrate concentrations in this watershed that was dominated by dairy farming and groundwater. Of these variables, the percentage of agricultural land emerged as the most significant factor.

Lessons learned from 20 y of monitoring suburban development with distributed stormwater management in Clarksburg, Maryland, USA

Urban development is a well-known stressor for stream ecosystems, presenting a challenge to managers tasked with mitigating its effects. For the past 20 y, streamflow, water quality, geomorphology, and benthic communities were monitored in 5 watersheds in Montgomery County, Maryland, USA. This study presents a synthesis of multiple studies of monitoring efforts in the study area and new analysis of more recent monitoring data to document the primary lessons learned from monitoring. The monitored watersheds include a forested control, an urban control with centralized stormwater management, and 3 suburban treatment watersheds featuring low-impact development and a high density of infiltration-focused stormwater facilities distributed across the watershed. Treatment watersheds were monitored before development, during construction, and after development. Monitoring was initiated to inform adaptive management of stormwater and impervious cover limits within the study area, with a focus on the impacts of distributed stormwater management. Results from our synthesis indicate that distributed stormwater management is advantageous compared with centralized stormwater management in numerous ways. Hydrologic benefits were greater with distributed stormwater infrastructure, demonstrating the ability to mitigate runoff volumes and peak flows and, for small storms, replicate predevelopment conditions. Baseflow temporarily increased during the construction phase in the treatment watersheds. Water-quality benefits were mixed, with declines in baseflow nitrate concentrations but limited changes to nitrate export and increases in specific conductance after development. Substantial topographic changes occurred during construction in the treatment watersheds, including changes within the riparian zone, despite riparian buffer protections. Ecological monitoring indicated that even though index of biotic integrity scores rebounded in some cases, sensitive benthic macroinvertebrate families did not fully recover in the treatment watersheds. Lessons learned from this synthesis highlight the importance of tracking multiple indicators of stream health and considering past land use and that more stormwater facilities distributed across the watershed is beneficial but cannot mitigate the effects of all urban stressors on aquatic ecosystems.

Baseflow nitrate dynamics within nested watersheds of an agricultural stream in Nebraska, USA

There is a need to evaluate high surface nitrate concentrations across agricultural watersheds, both spatially and temporally, to increase understanding of source and timing of nitrogen loads in streams and rivers. Bazile Creek is a high-nitrate stream originating in the agriculturally intensive Bazile Groundwater Management Area of Eastern Nebraska, USA. It is a gaining stream that receives groundwater with high nitrate concentrations originating from nonpoint sources. The objective of this study was to determine spatial and temporal variability of baseflow nitrate concentrations in Bazile Creek and its tributaries and to relate this variability to watershed characteristics. Surface-water nitrate samples were collected monthly from July 2018 through September 2019 from nine sites in the watershed and were analyzed for nitrate concentration. Average surface water nitrate-N concentrations within the watershed ranged from 2.7 to 15 mg L−1 and were significantly different between the sites (p < 0.05). Surface water nitrate-N concentrations varied seasonally in the main channel, recording the highest concentrations in winter (December-February, average = 14.4 mg L−1) when the discharge was minimum. High nitrate-N concentrations were observed in two of the five sampled tributaries, suggesting steady inputs of high-nitrate groundwater. The results of this study reveal substantial spatial variation in surface-water nitrate concentrations in the headwaters despite the close proximity of sampling sites. This study demonstrates that sampling tributaries along with the main channel of a stream is beneficial in determining nitrate inputs, variability and overall contaminant loading to a watershed.

Comparison of sediment and nutrient export and runoff characteristics from watersheds with centralized versus distributed stormwater management

Stormwater control measures (SCMs) are used to retain stormwater and pollutants. SCMs have traditionally been installed in a centralized manner using detention to mitigate peak flows. Recently, distributed SCM networks that treat runoff near the source have been increasingly utilized. The aim of this study was to evaluate differences among watersheds that vary in SCM arrangement by assessing differences in baseflow nutrient (NOx-N and PO4−) concentrations and fluxes, stormflow export of suspended sediments and particulate phosphorus (PP), and runoff characteristics. A paired watershed approach was used to compare export between 2004 and 2016 from one forested watershed (For-MD), one suburban watershed with centralized SCMs (Cent-MD), and one suburban watershed with distributed SCMs (Dist-MD). Results indicated baseflow nitrate (NOx-N) concentrations typically exceeded 1 mg-N/L in all watersheds and were highest in Dist-MD. Over the last 10 years in Dist-MD, nitrate concentrations in both stream baseflow and in a groundwater well declined as land use shifted from agriculture to suburban. Baseflow nitrate export temporarily increased during the construction phase of SCM development in Dist-MD. This temporary pulse of nitrate may be attributed to the conversion of sediment control facilities to SCMs and increased subsurface flushing as infiltration SCMs came on line. During storm flow, Dist-MD tended to have less runoff and lower maximum specific discharge than Cent-MD for small events (<1.3 cm), but runoff responses became increasingly similar to Cent-MD with increasing precipitation (>1.3 cm). Mass export estimated during paired storm events indicated Dist-MD exported 30% less sediment and 31% more PP than Cent-MD. For large precipitation events, export of sediment and PP was similar among all three watersheds. Results suggest that distributed SCMs can reduce runoff and sediment loads during small rain events compared to centralized SCMs, but these differences become less evident for large events when peak discharge likely leads to substantial bank erosion.

Channel complexity and nitrate concentrations drive denitrification rates in urban restored and unrestored streams

Stream restoration projects constructed in urbanizing watersheds provided an opportunistic setting to determine how channel bedform influenced nitrate removal by denitrification. We measured denitrification rates in streambed sediments seasonally at eight streams in North Carolina, USA to characterize the physicochemical drivers of nitrogen transformations in restored urban streams. Mean denitrification rates were highly variable (97±58 to 585±214μmolm−2h−1) and most significantly influenced by nitrate concentrations; baseflow nitrate concentrations ranged from 0.06 to 1.07mg/L. A multiple linear regression approach also revealed the importance of channel complexity, water depth and temperature, which together with nitrate concentrations explained 60% of the variability among sites and seasons. Controlling for nutrient limitation, we observed significantly greater denitrification rates in more geomorphically complex streams, particularly near grade control structures and in deep pools. However, these trends were not consistently observed across all sites, which suggest the added influence of watershed scale drivers (e.g., urban hydrology) as well as reach scale drivers including the design, construction and age of the restoration project.

Coupled hydrological and biogeochemical processes controlling variability of nitrogen species in streamflow during autumn in an upland forest

Autumn is a season of dynamic change in forest streams of the northeastern United States due to effects of leaf fall on both hydrology and biogeochemistry. Few studies have explored how interactions of biogeochemical transformations, various nitrogen sources, and catchment flow paths affect stream nitrogen variation during autumn. To provide more information on this critical period, we studied (1) the timing, duration, and magnitude of changes to stream nitrate, dissolved organic nitrogen (DON), and ammonium concentrations; (2) changes in nitrate sources and cycling; and (3) source areas of the landscape that most influence stream nitrogen. We collected samples at higher temporal resolution for a longer duration than typical studies of stream nitrogen during autumn. This sampling scheme encompassed the patterns and extremes that occurred during base flow and stormflow events of autumn. Base flow nitrate concentrations decreased by an order of magnitude from 5.4 to 0.7 µmol L−1 during the week when most leaves fell from deciduous trees. Changes to rates of biogeochemical transformations during autumn base flow explained the low nitrate concentrations; in-stream transformations retained up to 72% of the nitrate that entered a stream reach. A decrease of in-stream nitrification coupled with heterotrophic nitrate cycling were primary factors in the seasonal nitrate decline. The period of low nitrate concentrations ended with a storm event in which stream nitrate concentrations increased by 25-fold. In the ensuing weeks, peak stormflow nitrate concentrations progressively decreased over closely spaced, yet similarly sized events. Most stormflow nitrate originated from nitrification in near-stream areas with occasional, large inputs of unprocessed atmospheric nitrate, which has rarely been reported for nonsnowmelt events. A maximum input of 33% unprocessed atmospheric nitrate to the stream occurred during one event. Large inputs of unprocessed atmospheric nitrate show direct and rapid effects on forest streams that may be widespread, although undocumented, throughout nitrogen-polluted temperate forests. In contrast to a week-long nitrate decline during peak autumn litterfall, base flow DON concentrations increased after leaf fall and remained high for 2 months. Dissolved organic nitrogen was hydrologically flushed to the stream from riparian soils during stormflow. In contrast to distinct seasonal changes in base flow nitrate and DON concentrations, ammonium concentrations were typically at or below the detection limit, similar to the rest of the year. Our findings reveal couplings among catchment flow paths, nutrient sources, and transformations that control seasonal extremes of stream nitrogen in forested landscapes.

Influence of bedrock geology and tree species composition on stream nitrate concentrations in mid-appalachian forested watersheds

Although the large variations in nitrate export from forested watersheds have been at- tributed to a variety of natural and disturbance-related factors, baseflow nitrate concentrations in 49 mid-Appalachian forested watersheds were most strongly related to differences in bedrock geol- ogy. Within the mid-Appalachian region of Pennsylvania, Maryland and West Virginia, watersheds dominated by Pottsville and Allegheny sandstone (PVA), Catskill, Chemung, and Pocono shale and sandstone (CCP), and Mauch Chunk shale and Greenbrier limestone (MCG), respectively, exhibited significantly different low, intermediate, and high mean stream nitrate concentrations. Soil pH, soil percent N concentration (%N), soil C:N mass ratio, soil exchangeable Ca, watershed slope, and the occurrence of white ash (Fraxinus americana L.), sugar maple (Acer saccharum Marsh.), and eastern hemlock (Tsuga canadensis L.) were related significantly to bedrock geology type as well as stream nitrate levels. Other factors such as past land disturbances (fire and agriculture) and stand age (old- growth) typically were associated with only one bedrock geology type. However, within a bedrock geology type, past agriculture and the presence of old-growth forest may be important in explaining stream nitrate concentrations on an individual watershed basis. The basal area of black locust (Robinia pseudoacacia L.), a species that enhances soil nitrogen levels via nitrogen fixation, showed a moderate positive correlation with stream nitrate concentrations. Bedrock geology explained the most variation in winter (49%) and summer (32%) stream nitrate concentrations. Bedrock geology may have been a better predictor of stream nitrate concentrations than soil chemistry, because the geologic variation was better assessed at the regional scale of this study compared to soil chemistry, which varies at the micro-scale due to topographic, vegetation, microbial, and climatic influences. Results of this study suggest that bedrock geology is an important factor to consider when assessing forest nitrogen dynamics at a broad landscape scale.

Modelling nitrogen dynamics and distributions in the River Tweed, Scotland: an application of the INCA model

Abstract. The INCA (Integrated Nitrogen in Catchments) model was applied to the River Tweed in the Scottish Borders, a large-scale (4400km2), spatially heterogeneous catchment, draining a wide range of agricultural land-use types, and which contributes approximately 20% of UK river flows to the North Sea. The model was calibrated for the first four years' data record (1994 to 1997) and tested over the following three years (1998 to 2000). The model calibration and testing periods incorporated a high degree of variability in climatic conditions and river flows within the Tweed catchment. The ability of the INCA model to reproduce broad-scale spatial patterns and seasonal dynamics in river flows and nitrate concentrations suggests that the processes controlling first order variability in river water nitrate concentrations have been represented successfully within the model. The tendency of the model to overestimate summer/early autumn baseflow nitrate concentrations during dry years may be linked to the operation of aquatic plant uptake effects. It is, therefore, suggested that consideration be given to incorporating a spatially and temporally variable in-stream plant uptake term for the application of INCA to lowland eutrophic rivers. Scenarios to examine possible impacts of environmental change on nitrate concentrations on the Tweed are examined. These include the effects of (i) implementing different recommendations for fertiliser use and land use change under the Nitrate Sensitive Areas (NSA) Scheme and the Scottish Code of Good Agricultural Practice, (ii) worst case scenario changes linked to a dramatic reduction in livestock numbers as a result of a crisis in UK livestock farming and (iii) changes in atmospheric nitrogen deposition. Keywords: Nitrate, nitrogen, modelling, Tweed, INCA

Nitrate relationships between stream baseflow, well water, and land use in the Tomorrow-Waupaca Watershed.

We examined the use of stream baseflow water quality as a representative measure of mean ground water quality in the Tomorrow-Waupaca Watershed in central Wisconsin and the relationship between agricultural land use and watershed water quality. From 1997 to 1999, 38 stream sites were sampled for nitrate during winter and summer baseflow conditions. Some sites have been sampled during winter baseflow conditions since 1994. The land area contributing ground water to each stream sampling site was delineated, resulting in 38 sub-basins. In addition, over 3500 test results from private wells in the watershed were compiled and mapped using a Geographic Information System (GIS). Nitrate concentrations in stream baseflow and well waters were found to have strong positive correlation in the sub-basins of second order or higher. This indicates that stream baseflow may be valid for monitoring mean ground water quality in watersheds predominantly fed by ground water, where much of the stream nitrate is believed to originate from ground water. Analysis of seasonal variation in the stream data showed that winter nitrate concentrations were higher than summer concentrations, implying that winter stream monitoring may be more critical for the assessment of overall ground water quality in the watershed. We also found that, as the amount of agricultural land increased in each sub-basin, average nitrate concentrations in the well and stream waters also increased, suggesting a connection between agricultural land use and nitrate contamination of water resources in the watershed.

Chemical‐hydrologic interactions in the near‐stream zone

The chemical and hydrologic responses of a 9.9‐ha Pennsylvania hill‐land watershed to a typical summer storm event were determined and compared. Patterns and the relative magnitudes of NO3, NH4, total phosphorus (P), and orthophosphate (PO4) concentrations observed in seepage, surface runoff, storm flow, base flow, and rainfall fit those hypothesized from the storm hydrograph and associated water table responses observed in the near‐stream zone. Nitrate concentrations in seepage and base flow were similar and, typically, exceeded those in surface runoff, rainfall, and peak storm flow by 5–20 times. Conversely, NH4, total P, and PO4 concentrations in surface runoff from the seep zone and in peak storm flows exceeded those in seepage and base flow by 2–20 times. The findings, presented in a hydrologically based framework for this watershed, provide a conceptual model of how the near‐storm zone operates during and following storm events.

Seventy five years of arthritis research at the Strangeways Research Laboratory: 1912-87.

<h3>ABSTRACT:</h3> Nitrate contamination of shallow groundwater has been widely documented in association with agriculture in the Coastal Plain region of the Chesapeake Bay watershed. Elevated groundwater nitrate levels limit the use of shallow groundwater for human consumption and also result in elevated nonpoint source nitrogen (N) loads to Chesapeake Bay via elevated stream base-flow nitrate concentrations. This study investigated the effects of cereal grain winter cover crops on nitrate leaching rates, profile nitrate storage, and nitrate concentrations in shallow groundwater in two field-scale watersheds planted continuously in corn (Zea mays L) from 1984 through 1996. Winter-fallow conditions were maintained following the 1984 through 1987 growing seasons and cereal rye (Secale cereale L.) as a cover crop was planted immediately after grain harvest from 1988 through 1996. Cover crop effects on nitrate leaching rates also were evaluated in continuous no-till corn plots from 1990 through 1995. Nitrate leaching losses from the root zone and recharge of ‘shallow aquifers occurred primarily during winter months under conditions of law evapotranspiration’. The potential for nitrate leaching losses was determined primarily by the availability of nitrate in the root zone at the onset of the winter groundwater recharge period. Rye winter cover crops planted after corn harvest consistently reduced nitrate-N concentrations in root zone leachate to less than 1 mg/L during most of the groundwater recharge period, and reduced annual nitrate leaching losses by approximately 80% relative to winter-fallow treatments. Shallow groundwater nitrate-N concentrations under long-term continuous corn production decreased from the 10 to 20 mg/L range to less than 5 mg/L after seven years of cover crop use. Cover crops appeared to increase corn yields under adverse growing season conditions, but limited residual nitrate availability during the growing season relative to winter-fallow settings. Cover crop growth was generally N limited, suggesting that increased N inputs would have little effect on nitrate leaching, but would increase cover crop contributions to soil carbon pools.